FW: Tesla Technology Testing Toyota's iQ

FW: Tesla Technology Testing Toyota's iQ

Is that off the cuff or do you know how they used all those small cells?
There are some very good design decisions that went into it. Besides. lots of batteries are made from lots of internal cells, you
just don't always see them.

> Hopefully they'll use the good parts like the drive train and leave the dumb stuff like 6800 itty-bitty battery connections with the attracted complexity of multiple uP balancers out.
>

Re: FW: Tesla Technology Testing Toyota's iQ

Simply, the packs were from AC Propulsion. Parralleled in groups of 68 cells with copper end plates and I believe welded tabs. Then these are strung in series strings to get the required voltage. The idea is that the failure of one small cell (in the 68 cell module) will not significantly degrade the overall performance of the pack.

Additionally, building packs with the small 18650 commercial cells could help to keep battery costs down....eventually.

Is that off the cuff or do you know how they used all those small cells?
There are some very good design decisions that went into it. Besides. lots of batteries are made from lots of internal cells, you
just don't always see them.

> Hopefully they'll use the good parts like the drive train and leave the dumb stuff like 6800 itty-bitty battery connections with the attracted complexity of multiple uP balancers out.
>

Re: FW: Tesla Technology Testing Toyota's iQ

Similar concept. Yes their are 69 cells in parallel spot welded. But these 69 cells are in PARALLEL not SERIES and they each have a
PTC and fuse.

So now 1/69 of a module can fail dead short and you still have 68/69 capacity remaining

Kinda like disk RAID. RAIB? Reducdent Array of Inexpensive Batteries?

If they were in series I would agree with your 6800 points of failure concern.

As for the wasted space argument, I think you are missing an important point. The cylindrical cell is automatically protected from
swelling. and has a decent surface area ratio for cooling.

As cells get larger, they have swelling issues and loose surface area rapidly compared to volume.

Here is an excerpt from the white paper

"This cooling system design is especially effective because we have chosen to combine thousands
of small cells rather than several large ones to build an ESS, dramatically increasing the surface
to volume ratio. For example, with seven thousand 18650 cells the surface area is roughly 27
square meters. If there were an imaginary set of 20 much larger cube-shaped cells that enclosed
the same volume, the surface area would be only 3.5 square meters, more than seven times
smaller. Surface area is essential to cooling batteries since the surface is where heat is removed;
more is better. Also, because of their small size, each cell is able to quickly redistribute heat
within and shed heat to the ambient environment making it essentially isothermal. This cooling
architecture avoids hot spots which can lead to failures in large battery modules."

I think tesla has hit the nail on the head with thermal issues effects on lonjevity. I remember discussing this with JB at that
opening of the harris ranch chargeing station.

> Simply, the packs were from AC Propulsion. Parralleled in groups of 68 cells with copper end plates and I believe welded tabs. Then these are strung in series strings to get the required voltage. The idea is that the failure of one small cell (in the 68 cell module) will not significantly degrade the overall performance of the pack.
>
> Additionally, building packs with the small 18650 commercial cells could help to keep battery costs down....eventually.
>
> I look at it slightly differently, 6800 cells means 6800 potential failures.....and allot of wasted space.
>
> David
>
>
> ---------- Original Message ----------
> From: Jeff Shanab <[hidden email]>
> To: [hidden email]> Subject: [EVDL] FW: Tesla Technology Testing Toyota's iQ
> Date: Thu, 22 Jul 2010 13:01:58 -0700
>
> Is that off the cuff or do you know how they used all those small cells?
> There are some very good design decisions that went into it. Besides. lots of batteries are made from lots of internal cells, you
> just don't always see them.
>
>> > Hopefully they'll use the good parts like the drive train and leave the dumb stuff like 6800 itty-bitty battery connections with the attracted complexity of multiple uP balancers out.
>> >
>
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>

Re: FW: Tesla Technology Testing Toyota's iQ

On 7/23/2010 11:19 PM, Jeff Shanab wrote:
> Similar concept. Yes their are 69 cells in parallel spot welded. But
> these 69 cells are in PARALLEL not SERIES and they each have a PTC
> and fuse.
>
> So now 1/69 of a module can fail dead short and you still have 68/69
> capacity remaining

Maybe... A PTC does not open like a fuse; it goes to a higher
resistance. That means the shorted cell is still draining all the other
68 cells in parallel. The car can only sit for a certain length of time
without charging before *all* the cells will be totally drained, and
therefore go bad as well. How long this will take is unknown.

And, the PTC stays hot. It is dissipating heat continuously. How long
before that heat becomes a problem? Another unknown.

> If they were in series I would agree with your 6800 points of failure
> concern.

Yes; but then you would (presumably) have 69 strings in parallel.
Properly done, each of them could continue functioning with one string
disconnected due to a bad cell.

> As for the wasted space argument, I think you are missing an
> important point. The cylindrical cell is automatically protected
> from swelling. and has a decent surface area ratio for cooling.

I believe Tesla spent a lot of effort on cooling. But most Li packs I
see seem to ignore the thermal problem. They just pack them as tight as
physically possible, and don't worry about heat.

I think one reason for short laptop battery life is that they routinely
run them hot.

Re: FW: Tesla Technology Testing Toyota's iQ

> On 7/23/2010 11:19 PM, Jeff Shanab wrote:
>> Similar concept. Yes their are 69 cells in parallel spot welded. But
>> these 69 cells are in PARALLEL not SERIES and they each have a PTC
>> and fuse.
>>
>> So now 1/69 of a module can fail dead short and you still have 68/69
>> capacity remaining
>
> Maybe... A PTC does not open like a fuse; it goes to a higher resistance. That means the shorted cell is still draining all the other 68 cells in parallel. The car can only sit for a certain length of time without charging before *all* the cells will be totally drained, and therefore go bad as well. How long this will take is unknown.

Right, I think that is why each cell has both. The PTC cannot handle a shorted cell if the other cells are there to push on it, the
fuse will go.
The way I understand it a weak cell under load will attempt to reverse, the PTC will diminish the chance of reversing it or damage
from too many amps.
If it is pushed beyond that, like a shorted cell or serious cell reversal, the fuse will permanently remove it from the circuit.
But I think there may be an edge case where this fails. A dead module developing a partial short that never quiet blows the fuse.
>
> And, the PTC stays hot. It is dissipating heat continuously. How long before that heat becomes a problem? Another unknown.
Water cooling helps that. I think it is at least calculate-able. That may of been part of the SAE testing.
Just thinking out loud. Lets say the cell is partially shorted and saging to 1 V while it puts out amps that will drain it in 30
seconds. There is 2.8ah*3.2V or 9wh max in a cell. Lets say 100A and sag to 2V, It will be dead in 22 seconds while dissipating a
constant 200W.
Is the worst case is a totally full pack that is involved in a wreck and multiple pinched cells start shorting and feeding off the
pack. Or is it an almost dead pack that can stay jsut below the threshold of the fuses.
>
>> If they were in series I would agree with your 6800 points of failure
>> concern.
>
> Yes; but then you would (presumably) have 69 strings in parallel. Properly done, each of them could continue functioning with one string disconnected due to a bad cell.
Now separate strings in parallel would definitly be more ideal, but I am sure a compromise had to be struck to get weight and
complexity down. (They went with strings of length 1; with a fuse and ptc in series, right?)

None the less, it is the motivation for one of my designs. 144V 10ah modules using the 38120 headway cells or the 42110 HiPower
cells. These modules would be stand alone but could be combined to create a 144 or 288 pack that charges at 144V. In this way they
could be truly redundant and the BMS would scale with application. Also, expanding the system later may be made easier as you could
add capacity without increasing system voltage.

>
>> As for the wasted space argument, I think you are missing an
>> important point. The cylindrical cell is automatically protected
>> from swelling. and has a decent surface area ratio for cooling.
>
> I believe Tesla spent a lot of effort on cooling. But most Li packs I see seem to ignore the thermal problem. They just pack them as tight as physically possible, and don't worry about heat.
>
> I think one reason for short laptop battery life is that they routinely run them hot.

Agreed on both counts. At first I thought it was overkill, but they are showing data that better temp management = longer life. And
that is certainly no secret even with lead.

On a side note, I have even seen ads for those laptop coolers stating increased battery life, so I think that is starting to become
a mainstream idea (not that becoming a mainstream idea means anything)